The invention finds application in the recovery of bitumen from bituminous sands in the process generally known as the hot water process. More particularly, the invention describes a process for obtaining bitumen from oversized material rejected from the conditioning vessel in which bituminous sand is slurried with hot water in said process.
A large proportion of the world's known hydrocarbon reserves exists in the form of bituminous sand, commonly referred to in the industry as tar sand and hence so referred to hereinafter. One large deposit of this material is found in Alberta, Canada, in an area traversed by the Athabasca River. The tar sand is believed to exist in the form of water-wet grains of sand sheathed in films of bitumen. The bitumen is a valuable hydrocarbon material because, by suitable methods of upgrading and refining, it can be converted to refined liquid and gaseous products useful for domestic and industrial purposes.
The sand particles in the deposit are generally of such a size as to be retained by a screen of 325 mesh, although clays and slits having much smaller particle size are also present. The quantity of bitumen as a proportion of the total composition is typically of the order of 12% by weight.
Many methods have been proposed for recovering the hydrocarbons from tar sand. These include removing the hydrocarbons from mined tar sand by solvent extraction, or by separation with hot water, or by direct thermal treatment. Still other methods, the so-called in situ methods, dispense with the mining step and render the hydrocarbons recoverable by treating the tar sand (usually with heat) in place.
Tar sand presents considerable difficulties in the recovery of the bitumen partly of the very fine nature of some of the mineral solids and partly because of the intimate admixture of the mineral solids and the bitumen. At present the only commercially viable method is the hot water process. The aim of this process is to separate the bitumen from tar sand and in such a form as to allow further purification to remove substantially all associated water and mineral solids. According to the hot water process, mined tar sand is fed to a rotating conditioning vessel, ususally known as the "tumbler", where it is treated with hot water. The water is commonly heated by steam, added directly to the tumbler. Sodium hydroxide is also added in such quantities as to raise the pH of the mixture to about 9.0. Feed materials are fed to the tumbler in the following typical proportions by weight: tar sand, 3250; water 610.3; sodium hydroxide solution (at a specific gravity of 1.22) 4.06.
On emerging from the tumbler, the conditioned slurry is screened and diluted with hot flood water to the extent that the composition of the diluted slurry is typically 7.30% butumen, 42.52% water and 50.18% mineral solids. This mixture is then subjected to bitumen separation. This involves treating it in a flotation vessel, so designed as to allow the bitumen to rise as a froth typically containing 66.4% bitumen, 8.9% mineral solids, and 24.7% water. A further yield of bitumen is obtained from the middlings phase of the flotation vessel and, after combination, the combined froths are treated with a diluent hydrocarbon and advanced to centrifuges to produce purified hydrocarbon mixture which, in turn, may be distilled to remove the diluent so that the resulting free bitumen may then be converted to hydrocarbons of lower molecular weight, collectively known as synthetic crude oil, by thermal treatment.
The slurrying step performed in the tumbler dislodges the bitumen from the tar sand. The slurrying time and mixing intensity can be altered according to the type of tar sand feed being introduced to the tumbler. However the conditions to which the tar sand is subjected affect not only the degree of tar sand digestion, but also the ease with which the bitman may be isolated from the mixtures in the subsequent separatory steps. Too vigorous treatment or too extended a residence time leads to over-conditioning, which interferes with and reduces the recovery of bitumen in the separatory steps. To avoid over-conditioning, the tumbler must be operated in such a way that some of the harder, more resistant lumps of tar sand are allowed to pass through the tumbler substantially unaffected by the hot water treatment and, on being rejected by the exit screens mentioned above, are removed from the process, thus representing loss of bitumen product. In addition the exit screens also reject oversized rocks and other debris and, as these are coated with a layer of bitumen-containing material, their rejection also contributes to loss of bitumen from the process.
Typically the bitumen content of the rejects is 2.00% by weight and represents about 0.5% of the bitumen in the tar sand feed.